1. Field of the Invention
The present invention relates to a modem chip for a digital communication system, and more particularly to an apparatus and a method for actively adjusting the quantization interval of signals inputted to a decoder in a digital communication system.
2. Description of the Related Art
As generally known in the art, conventional digital communication systems, particularly CDMA-type digital communication systems based on IS-2000, support voice services alone. However, the rapid development of mobile communication service technology and increasing user demand require that they also support data services in addition to voice services.
For example, an HDR (High Data Rate) mobile communication system is adapted to solely support a high-rate data service.
The receiver of mobile communication systems demodulates multi-path signals, which are received via different paths, and combines the modulated signals. The receiver includes at least two fingers for separately receiving RF (Radio Frequency) signals. The receiver allocates the multi-path signals, which have different time delays after going through different paths, to respective fingers, which then estimates the channel gain and phase, demodulates RF signals, and creates traffic symbols. The created traffic symbols are combined to improve the signal-receiving quality based on a time diversity effect.
FIG. 1 is a block diagram showing a conventional receiver. For clarity, only components related to decoder input are shown in the drawing.
Signals are received via an antenna and are mixed with carrier frequencies. Then, the signals undergo down-conversion, pass through an ADC (Analog-to-Digital Converter), which is not shown in the drawings, and are input to a rake receiver of a digital baseband stage. The rake receiver of the digital baseband stage includes a number of fingers 110 and 120 and a combiner 130. Each finger 110 and 120 receives data from a PN sequence generator (not shown) and despreads the data so that it has the same PN sequence as used by the base station. A Walsh sequence generator (not shown) multiplies the resulting data by a Walsh sequence, which corresponds to a channel to be demodulated. An accumulator (not shown) accumulates the resulting sequence as much as the symbol length so as to conduct Walsh discovering. At the same time, a channel estimator (not shown) estimates the current channel condition by using a pilot channel. A conjugation unit (not shown) obtains a conjugate from the channel estimation value. Multipliers 111 and 121 conduct complex multiplication with regard to the accumulated symbols as channel compensation. The demodulated symbols are output to the combiner 130, which combines the output from each finger and outputs it to the decoder stage.
The dynamic range of signals input to the decoder greatly varies depending on the signal modulation type, the wireless channel environment, and the number of times the packet codeword is repeated. Considering these varying factors, the dynamic range of decoder input is conventionally set to be large enough to accommodate the entire dynamic range of signals input to the decoder, when the receiver of the terminal modem is designed. Therefore, the conventional quantizer 140 must consider all of the modulation type, the amount of change of the wireless channel environment, and the maximum number of times the packet codeword is repeated, i.e. the worst case, when determining the dynamic range of signals input to the decoder. In addition, the quantizer 140 determines the quantization interval based on the number of effective bits used by the decoder. However, the quantizer 140 has a problem in that, since the quantization interval is determined solely against the worst case, the quantization cannot be optimized in a normal case (i.e. when the case is not the worst case). This lowers the signal-receiving performance of the decoder and degrades the decoder performance.